The goal of the proposed research is to understand at the molecular level how microtubules are organized in the cytoplasm. The work is based on results from our laboratory suggesting that different minor microtubule components play a role in determining the assembly and interactions of microtubles. In one set of experiments, family of associated proteins - the chartins - are analyzed to identify the relationship between structure and function. That work grows out of previous results suggesting that different covalent forms of this protein are associated with different differentiated microtubule organelles. A direct approach toward identifying the structural differences will be taken, and experiments designed to test the function of those differences are proposed. In a second set of experiments, the molecular determinants of an extraordinary microtubule structure, the marginal band, will be identified. This work grows out of results suggesting that the microtubule associated proteins in this case interact with other elements of the cytoskeleton. We will attempt to generalize the results of this work to other normal cells, and determine if these interactions change in the transition from the proliferative to the differentiated state. These experiments may give more insight into the mechanisms by which cells specify their differentiated morphology, and how that organization is disrupted during transformation.